Patent Document 1 (shown below) discloses an apparatus for calculating a cylinder intake air amount using an engine rotational speed, an intake pressure, and a charging efficiency (volumetric efficiency). In this apparatus, an air-fuel ratio learned value for correcting changes in the charging efficiency is calculated according to a detected air-fuel ratio, and the cylinder intake air amount is calculated using the charging efficiency corrected with the air-fuel ratio learned value.
Patent Document 2 (shown below) discloses an apparatus for calculating a volumetric efficiency equivalent value which indicates a volumetric efficiency of the engine, and calculating a cylinder intake air amount using a present calculated value and a preceding calculated value of the volumetric efficiency equivalent value, and a detected intake fresh air amount. In this apparatus, the volumetric efficiency equivalent value is calculated according to a coefficient f(Ne) depending on the engine rotational speed, a coefficient G(Regr) depending on the exhaust gas recirculation rate, an intake pressure, and an atmospheric pressure.